The present invention relates to optical instruments and, more particularly, to an optical time-domain reflectometry system including a novel pulse source.
Optical time-domain reflectometry (OTDR) is an approach to characterizing objects by measuring the duration between transmission of a pulse and its detection upon reflection. For example, OTDR can be used to evaluate photonic instruments as well as bulk optical devices such as photographic lenses. A laser pulse directed into a multi-element lens can result in a echo signal with a time-varying intensity, intensity corresponding to reflectivity and time corresponding to distance from the pulse source. Accordingly, the echo signal can be analyzed to determine how much each lens surface reflects light intended to be transmitted therethrough. Moshe Nazarathy et al. in "Real-Time Long Range Complementary Correlation Optical Time Domain Reflectometer", IEEE Journal of Lightwave Technology, Vol. 7, No. 1, Jan. 1989, and the references cited therein describe several OTDR systems.
A typical OTDR system uses a transistor to switch a semiconductor laser to generate optical pulses. A counter can be started upon transmission and stopped upon detection of a generated pulse to measure the intervening duration. This duration can be converted to a path distance to locate the source of each reflection. The precision to which distances can be resolved is limited by the pulse-width of the pulse as transmitted as well as by dispersion that occurs between transmission and detection.
By switching a laser diode, optical pulses of 50 picoseconds (ps) are readily attainable. This is sufficient for analyzing simple lenses with elements spaced more than a centimeter apart. High speed laser diodes have been demonstrated which provide 10 ps temporal resolution, sufficing for resolution on the order of a few millimeters.
Modern photographic lenses, especially zoom lenses, can utilize fifteen or more elements. To provide a lens which is reasonably compact, the lens elements must be tightly spaced, typical spacings being less than a few millimeters. Resolving reflections from the surfaces of such closely spaced elements can require OTDR systems with initial pulse widths on the order of 1 ps.
The desired resolution can be obtained by employing high-powered pulsed lasers or bulk optical compressors, as disclosed by Edmond B. Treacy in "Optical Pulse Compression With Diffraction Gratings", The Journal of Quantum Electronics, Vol. QE-5, No. 9, September 1969. However, the power and weight of these optical sources make them expensive and awkward. The expense and power requirements can preclude their use in many applications requiring portability. What is needed is an economical OTDR system which can characterize optical components with very high spatial resolution while having modest bulk and power requirements.